Heater modules having an outer cylindrical heat exchanger and a burner positioned in a central cavity thereof have been used in the prior art. These heater modules may have end manifolds that have chambers for circulating a liquid back and forth through tubes in the heat exchanger. For example, each chamber may have two or more tubes communicating with it and the liquid from the opposite end enters the chamber from one tube and returns through the other. In other words, the chamber function as elbow conduits for two tubes or sets of tubes. In one such heater module, the manifold at one end is manufactured by stamping a unitary piece of steel such that there is a flat inner area that covers the end of the central cavity and a peripheral trough. The chambers are formed in the trough by attaching radial baffles therein. An annular plate with apertures for the tubes covers the trough and the structure so formed is made water tight by copper plating and brazing. In the flat inner area or disk, there is a circular opening through which the burner inserts into the cavity. The hot gases of combustion from the burner then pass outwardly through the central cavity and flow through the heat exchanger. Manifolds such as described above may be subject to cracking at or near the region where the annular plate attaches to the trough.